It is more particularly applicable to exploring or exploiting underground reservoirs of oil or gas, or of hot or cold water, in circumstances which make it impossible or unadvisable to reach the underground target point by means of a rectilinear well (whether vertical or sloping). An underground obstacle, even if known about in advance, may lead to the inclincation or azimuth, for example, of a well being changed at a point situated at some depth below the surface. The same applies when a length of the drilled well is to run along a shallow stratum which may slope to a greater or lesser extent or which may be horizontal.
In such cases, bends in the well must be included in the original drilling plan.
In addition, even when drilling vertically, it may happen that the rocks which are encountered cause an unwanted deflection of the drilling axis.
Such an unwanted deflection must be compensated by a deliberate deflection which may be referred to as a "correction bend". Regardless of the reason which makes such operations necessary, bending operations begin with the operation of measuring the inclination and the azimuth or bearing angle of the axis of the portion of the well being drilled, and also the position which has been reached, which measurement is the result of "dead reckoning" based on the lengths, the slopes, and the bearings, of all the lengths of the well drilled so far. Once these measuring operations have been performed, a bend needs to be made. A drilling method for making such a bend comprises a succession of distinct drilling stages, namely at least one stage of curved drilling together with a plurality of rectilinear drilling stages during each of which a rectilinear length of well is drilled along a rectilinear axis. The, or each, stage of curved drilling is interposed between two stages of rectilinear drilling. While curved drilling is taking place, a curved length of well is drilled having a curved axis which is substantially in the form of an arc of a circle, and this arcuate axis runs on from the axis of the preceding rectilinear length of well. Similarly, the axis of the following rectilinear length of well runs on from the axis of the curved length.
During each of these drilling stages, a drilling tool is used which has an axis running along a top-to-bottom direction (or a back-to-front direction if drilling is taking place horizontally), which tool is suitable for drilling downwardly (forwardly) and optionally sideways into the rock at the bottom end of the well. A bottom motor is fixed above (behind) said tool for driving it and has an axis which lies in the extension of the tool axis. Tubes, known as "drill-stems" in the art, are fixed above (behind) said motor one after the other and constitute a "string" extending to the mouth of the well and serving to convey drilling mud to the bottom of the well under pressure, which mud also provides the energy required for driving the motor. The drilling string is also used for remotely controlling from the surface the pressure with which the tool presses against the rock, for optionally rotating the entire assembly, and for supporting the tool and the motor whenever they are raised or lowered along the well.
(It must be understood that throughout this specification the terms "up" and "down" or "top" and "bottom", etc., may need to be replaced with "backwards" and "forwards" or "back" and "front" etc., respectively, in the event that the well is advancing horizontally).
The drilling stems have axes which extend the motor axis and which extend one another. The number of stems is progressively increased by adding new stems as the well gets deeper. The diameter of the motor and the diameter of the stems are less than the diameter of the normal well as drilled by the tool, and the stems are sufficiently flexible for a drilling string constituted by the above-described components to be capable of following curved lengths of well.
It is known that curved drilling stages can be provided by incorporating a bending device in the drilling string at the top of the down-hole motor which drives the drilling tool. The bending device is a bendable coupling which causes the tool axis to slope relative to the axis of the remainder of the string. The down-hole motor itself may also be bent. Another bending method consists in using excentric stabilizers. Such a stabilizer constitutes at least one asymmetrical radial projection whose end is at a slightly greater distance from the axis of the string than the normal radius equal to one-half of the normal well diameter. It thus presses against the wall of the well being drilled and offsets the axis of the down-hole motor from the axis of the well with the string bending elastically, thereby providing an angular deflection of the drilling direction of the tool relative to the axis of the length of well being drilled, and thus giving said length the desired curved shape.
The down-hole motor is generally a turbine or a Moineau-type motor.
In some known methods, the entire drilling string is raised to the surface each time a rectilinear length is to be drilled beyond a curved length, with the string being raised so that the bending device such as a bent coupling, a bent motor or excentric stabilizers may be removed. Similarly, the entire string must be raised to the surface each time a new bend is to be made since the bending device must then be put back into position. These maneuvers of raising and lowering the drilling string require each of the drilling stems to be successively removed from or added to the string. Such maneuvers thus occupy a major portion of the time required for providing a bend, which time includes the above-mentioned measuring operations and drilling the curved length of well for obtaining the desired change of direction. The time taken constitutes one of the major items in the cost of drilling a well.
In accordance with another known method using excentric stabilizers, a rectilinear length of well is obtained beyond a curved length by rotating the entire string. The tool then drills a hole of enlarged diameter. This method avoids the onerous operation of raising and lowering the drilling string, but it suffers from the drawback of subjecting the down-hole motor to large bending stresses.
This second known method is described, for example, in Shell's European patent application published as EP-A 2, No. 0 085 444.
Other known methods use a controllable bending device, in particular a controllable bend coupling constituting a length of the drilling string just above the down-hole motor. Such a device is controllable to take up a bent configuration during stages for which a bent length of well is being drilled, or a rectilinear configuration during stages for which a rectilinear length of well is being maintained or re-established. The device comprises two successive sleeves capable of pivoting relative to each other about a pivot axis which causes the axes of both sleeves to pivot through the same small angle. Pivoting is controlled either by means of an axial displacement of the top portion of the column or else by means of a pivot motor driving a gear train as described in U.S. Pat. No. 4,303,135 (Benoit).
Such controllable bending devices suffer from the drawback of being complex.
Preferred implementations of the present invention enable curved drilling stages and rectilinear drilling stages to follow one another without losing time and while allowing the drilling tool to operate under good conditions throughout, this being achieved by means of a controllable bending device which is similar than that described in the above-mentioned Benoit patent.